Fix: don't access packet header for stream_id and stream_instance_id getters

The stream ID and stream instance ID are invariant for a stream, so
there is no point reading them from the packet header currently owned by
the consumer (between get/put subbuf).

Actually, the consumer try to access the stream_id from the live timer
when sending a live beacon without getting the reader subbuffer first.
Doing so is racy against producers. In typical live scenarios
(non-overwrite channels), the producers will always write the same
stream id and stream instance id values at the same header offsets,
which will "work", except for the initial state of an empty buffer:
the value "0" will be returned (erroneously).

For the less frequently used scenario of a live session with "overwrite"
channels, this is handled by issuing a CHAN_WARN_ON, which disables
tracing for the channel, and prints warning to the consumerd console
when running consumerd with LTTNG_UST_DEBUG=1.

In the case where a ring buffer does not have any data ready, it makes
no sense to try to get a subbuffer for reading anyway, so the approach
was broken.

So return the stream id and stream instance id from the internal
data structures rather than reading it from the ring buffer.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

diff --git a/liblttng-ust/lttng-ring-buffer-client.h b/liblttng-ust/lttng-ring-buffer-client.h
index 0fae88787c8b263a93f29b865153f22aff7ca3cf..ab472352f96898625db0662b9399e5762088d897 100644 (file)
--- a/liblttng-ust/lttng-ring-buffer-client.h
+++ b/liblttng-ust/lttng-ring-buffer-client.h
@@ -580,12 +580,11 @@ static int client_stream_id(struct lttng_ust_lib_ring_buffer *buf,
                struct lttng_ust_shm_handle *handle,
                uint64_t *stream_id)
 {
-       struct packet_header *header;
+       struct channel *chan = shmp(handle, buf->backend.chan);
+       struct lttng_channel *lttng_chan = channel_get_private(chan);
+
+       *stream_id = lttng_chan->id;
 
-       header = client_packet_header(buf, handle);
-       if (!header)
-               return -1;
-       *stream_id = header->stream_id;
        return 0;
 }
 
@@ -616,10 +615,8 @@ static int client_instance_id(struct lttng_ust_lib_ring_buffer *buf,
                struct lttng_ust_shm_handle *handle,
                uint64_t *id)
 {
-       struct packet_header *header;
+       *id = buf->backend.cpu;
 
-       header = client_packet_header(buf, handle);
-       *id = header->stream_instance_id;
        return 0;
 }